Let's get to know ELLIPTICAL GALAXIES

 

An elliptical galaxy, full of dark lanes of gas, likely formed in the merger of two other galaxies. Elliptical galaxies host less (or no) star birth than spiral galaxies like the Milky Way. (Image credit: NASA, ESA and The Hubble Heritage Team (STScI/AURA))

WHAT ARE ELLIPTICAL GALAXIES? 

Elliptical galaxies are the most abundant type of galaxies found in the universe but because of their age and dim qualities, they're frequently outshone by younger, brighter collections of stars.

Elliptical galaxies lack the swirling arms of their more well-known siblings, spiral galaxies. Instead, they bear the rounded shape of an ellipse, a stretched-out circle.

One of the most famous elliptical galaxies is Cygnus A, which is located roughly 600 million light-years from Earth and is an extremely bright radio source. Cygnus A is not only well-known to astronomers, but has a place in science fiction; it was featured in the 1985 novel "Contact," a Carl Sagan story that later inspired a Hollywood movie of the same name.

CLASSIFICATION & CHARACTERISTICS 

In 1926, Edwin Hubble devised a system to classify galaxies. Known as the Hubble sequence, or the "Hubble tuning-fork," it organizes galaxies based on their shape. Under this organization, elliptical galaxies are classified by how stretched out they are. Galaxies classified as E0 appear to be almost perfect circles (remember, a circle is an ellipse), while those listed as E7 seem much longer than they are wide. A galaxy's appearance is related to how it lies on the sky when viewed from Earth. A galaxy having the E7 shape but seen head on would appear as an E0, for instance, because observers would not see its stretched shape, which lies "behind" it.

Elliptical galaxies have a broader range in size than other types of galaxies. The smallest are dwarf elliptical galaxies, which can be less than 10 percent of the size of the Milky Way. But ellipticals can also stretch to more than a million light-years across, and contain more than ten trillion stars. M87, identified as one of the largest galaxies in the universe, is classified as an E0 elliptical galaxy.

Astronomers have identified more spiral galaxies than ellipticals, but that's simply because the spirals are easier to spot. While spiral galaxies are bright, elliptical galaxies are dim. Spiral galaxies are hotbeds of 4 have identified more spiral galaxies than ellipticals, but that's simply because the spirals are easier to spot. While spiral galaxies are bright, elliptical galaxies are dim. Spiral galaxies are hotbeds of star formation, but elliptical galaxies aren't nearly as prolific because they contain less gas and dust, which means fewer new (and brighter) stars are born. The existing stars inside an elliptical galaxy tend to be older, giving off more red light than younger stars.

So, why do astronomers think elliptical galaxies dominate the sky? Because when specific regions of the sky are studied in depth, more elliptical galaxies appear. Astronomers think such counts are consistent throughout the universe.

HISTORY & FORMATION 

Milky way

Because elliptical galaxies contain older stars and less gas, scientists think that they are nearing the end of the evolutionary line for galaxies. The universe is a violent place, and collisions between galaxies are frequent — indeed, the Milky Way is due to crash into the Andromeda Galaxy in a few billion years. When two spirals collide, they lose their familiar shape, morphing into the less-structured elliptical galaxies.

A supermassive black hole is thought to lie at the center of these ancient galaxies. These gluttonous giants consume gas and dust, and may play a role in the slower growth of elliptical galaxies.

Born from collision, elliptical galaxies are more commonly found around clusters and groups of galaxies. They are less frequently spotted in the early universe, which supports the idea that they evolved from the collisions that came later in the life of a galaxy.

Research in just the past few years has revealed quite a bit about the evolution of elliptical galaxies.

A 2014 study showed that supermassive black holes might be stopping star formation in elliptical galaxies, which would help explain why there are so few new stars in ellipticals. Previously, astronomers thought ellipticals lacked the cold gas necessary for star formation, but the new study showed that there is actually a lot of cold gas in ellipticals, it's just blasted away by supermassive black holes in the region.

In 2015, researchers discovered that some compact ellipticals might lose most of their mass through galaxy interactions, which has large implications for what happens when galaxies merge and collide.

Galaxy mergers with ellipticals could even produce weird hybrids, such as the Sombrero Galaxy. A 2012 examination by NASA's Spitzer Space Telescope found that Sombrero is made up of one galaxy inside of another; the study reclassified Sombrero to an elliptical galaxy with a flat disk inside of it.

Such examinations sometimes also reclassify elliptical galaxies into other types, such as with UGC 1382 in 2016. Scientists determined that the galaxy was not an elliptical, but a sort of strange combination of cosmic pieces that came about through various mergers. Scientists are now interested in how many other similar galaxies reside in our universe.

SIZES AND SHAPES 

Elliptical galaxies vary greatly in both size and mass with diameters ranging from 3,000 light years to more than 700,000 light years, and masses from 105 to nearly 1013 solar masses.

This range is much broader for this galaxy type than for any other. The smallest, the dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. Most of these small galaxies may not be related to other ellipticals.

The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major (a) to the minor (b) axes of the galaxy's isophotes:

Elliptical galaxies vary greatly in both size and mass with diameters ranging from 3,000 light years to more than 700,000 light years, and masses from 105 to nearly 1013 solar masses.

This range is much broader for this galaxy type than for any other. The smallest, the dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. Most of these small galaxies may not be related to other ellipticals.

The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major (a) to the minor (b) axes of the galaxy's isophotes:

Thus for a spherical galaxy with a equal to b, the number is 0, and the Hubble type is E0. While the limit in the literature is about E7, it has been known since 1966 that the E4 to E7 galaxies are misclassified lenticular galaxies with disks inclined at different angles to our line of sight. This has been confirmed through spectral observations revealing the rotation of their stellar disks. 

Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated.

It is sometimes said that there are two physical types of ellipticals: the giant ellipticals with slightly "boxy"-shaped isophotes, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion); and the "disky" normal and dwarf ellipticals, which contain disks. 

This is, however, an abuse of the nomenclature, as there are two types of early-type galaxy, those with disks and those without. Given the existence of ES galaxies with intermediate-scale disks, it is reasonable to expect that there is a continuity from E to ES, and onto the S0 galaxies with their large-scale stellar disks that dominate the light at large radii.

Dwarf spheroidal galaxies appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies.

At the large end of the elliptical spectrum, there is further division, beyond Hubble's classification. Beyond gE giant ellipticals, lies D-galaxies and cD-galaxies. These are similar to their smaller brethren, but more diffuse, with large haloes that may as much belong to the galaxy cluster within which they reside than the centrally-located giant galaxy.

EVOLUTION

Multiple nebulae

It is widely accepted that the merging of large but non-giant galaxies, due to gravitational attraction, plays a major role in shaping the growth and evolution of elliptical galaxies. Such major galactic mergers are thought to have been more common at early times. The Milky Way galaxy, depending upon an unknown tangential component, is on a four- to five-billion-year collision course with the Andromeda Galaxy.

 It has been theorized that an elliptical galaxy will result from a merger of the two spirals. (Note that minor galactic mergers involve two galaxies of very different masses; it is well established that minor mergers occur in galaxies that are neither giant nor ellipticals – such as our own Milky Way galaxy.)

It is believed that black holes may play an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting star formation.

STAR FORMATION 

The traditional portrait of elliptical galaxies paints them as galaxies where star formation finished after an initial burst at high-redshift, leaving them to shine with only their aging stars. Elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. In spirals, this blue color emanates largely from the young, hot stars in their spiral arms. Very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a reasonable proportion (~25%) of early-type (E, ES and S0) galaxies have residual gas reservoirs and low level star-formation.